Semiconductor memory circuits are used in many applications to store data. Most read-write memory circuits are volatile, that is, the data state stored in the memory circuit is lost when power is removed from the circuit. Thus, a system which has a completely volatile memory will lose all the data stored in the memory if power is temporarily lost from the system. In many applications the possibility of losing stored data from a volatile memory is a serious handicap. For certain applications the critical data can be protected from loss by the use of electrically programmable read only memories or by transferring the data into a nonvolatile storage medium such as magnetic disk or tape. But in many applications there are rapid and frequent changes in the state of the data such that it is extremely difficult to save the data in the event that circuit power is lost. To meet this problem there have been developed memory cells which combine a standard volatile memory circuit together with a nonvolatile memory element. Such a circuit is shown in U.S. Pat. No. 4,128,773. These circuits operate in the usual manner for conventional operation, but upon command the data state in the volatile element is shifted into the nonvolatile element. The power for the circuit can then be removed for an extended period of time. After the power is restored, the data state in the nonvolatile circuit is transferred into the volatile circuit element to return the system to the original data state condition.
Although the combination of volatile and nonvolatile memory circuits, also referred to as a "shadow RAM cell," have functioned successfully, the need for extensive circuitry substantially reduces the data density for a memory system. Therefore, in view of the requirements for nonvolatile backup and maximum memory density, there exists a need for a nonvolatile random access memory cell which can reliably store and recall data while having a minimum area configuration or have increased margin and reliability with a similar area as compared to prior art devices.